The world's most used construction materials are partially or completely constructed of cementitious materials, specifically, concrete. Concrete generally refers to a mixture of natural and/or artificial aggregates, such as sand and either a gravel or a crushed stone, which are held together by a binder of cementitious paste to form a highly durable building material. The paste is typically made up of a hydraulic cement, such as Portland cement, and water and may also contain one or more chemical admixtures as well as supplementary cementing materials, such as fly ash, ground granulated blast furnace slag, and silica fume.
In the late 1970's ultra-high performance concrete (UHPC) was seen as breakthrough in the materials industry having an increased compressive strength in excess of 120 MPa and increased durability compared to other high performance concretes known at that time. UHPC is also known as reactive powder concrete (RPC) or densely packed cementitious materials commonly also called densified systems containing homogeneously arranged ultrafine particles (DSP materials). Regardless of the variety of the names for this type of concrete and its compositions, these cementitious materials are formed by the hydration of densely packed particles of an inorganic binder in combination with ultrafine particles of a second, less reactive or non-reactive filler, as well as some additional particulate materials. It is understood by those skilled in the art that DSP materials give rise to hitherto unattainable mechanical qualities, including strength, density, and durability (examples include EP 0 010 777 B2, EP 0 269 71 5 B1, EP 0 042 935 B2, U.S. Pat. No. 4,979,992).
Inorganic binders are materials produced by mechanical comminution of coarser intermediates such as clinker and granulated blast furnace slag, and generally comprise particles ranging in sizes between 1 μm to 100 μm. Even when such particles are packed to a theoretical maximum degree, empty spaces remain that cannot be filled with inorganic binder particles in this size range. The binder most commonly utilized in producing UHPC's is Portland cement. Upon mixing with water, the empty spaces between the inorganic binders become filled with liquid. However, it is known in the art that the amount of water is reduced when the empty spaces between the inorganic binders are filled with a second material having smaller particle sizes than the inorganic binders. For example, U.S. Pat. No. 5,522,926 discloses the use of silica fume, specifically microsilica, an ultrafine material most commonly used to fill the voids between the inorganic binders. Such cementitious systems having two particle sizes exhibit a higher degree of packing, a lower porosity, a higher strength and lower permeability than conventional concrete.
The optimization of the granular mixture for the enhancement of compacted density, efficient micro-reinforcement, and more recently nano-reinforcement, as well as the optimal usage of superplasticizer, are among the most important factors defining strength and workability of UHPC. The uniform distribution of the cement-silica fume particles is critical in the production of a dense strong concrete matrix. Concretes have been designed having remarkable compressive strength and fracture energy based on the principle of optimizing mortar composition with the purpose of maximizing the matrix density of concrete (U.S. Pat. Nos. 5,522,926; 6,080,234; 7,901,504; 8,303,708). Furthermore, others have focused on maximizing the mortar packing density (U.S. Pat. Nos. 7,744,690 and 8,016,938).
Taking into account the critical importance of increased particle packing for making a strong concrete, many mathematical models have been developed, for example by Shakhmenko et al. Concrete Mix Design and Optimization, 2nd Int. PhD Symposium in Civil Engineering 1998 Budapest, 1-8 and by Yu et al., Mix design and properties assessment of Ultra-High Performance Fibre Reinforced Concrete (UHPFRC), Cement and Concrete Research 56 (2014) 29-39 for calculating concrete compositions having maximum particle packing density, and numerous software programs, based on these mathematical models have been created for calculating such optimum compositions.
The mathematical models which attempt to calculate ideal concrete compositions having maximum particle packing density are based on the theory that the powders are “ideal”, meaning that powders are loose and do not have interconnected agglomerated particles. According to this “ideal” powder concept, upon mixing silica fume powders with larger inorganic binders, the ultrafine microsilica particles are uniformly arranged in the voids between coarser cement particles, together creating a densely packed matrix. However, in actual practice, silica fume particles have a tendency to aggregate. The smaller the particle size, the more aggregation. In current systems, ultrafine silica fume materials aggregate and nano-particles severely aggregate. Both ranges of particle size aggregate, creating densely packed clumps, which reduces the benefits associated with silica fume having small particle sizes. Particle aggregation prevents uniform distribution of the silica fume particles from being achieved, and the density, as well as other properties of the concrete matrix is jeopardized in various ways, including but not limited to resulting in higher porosity, lower strength and durability and higher permeability. Although there are numerous modern multi-component particle packing models, which take into account the interaction between different components, the basic mathematical equations of almost all particle packing models are the same and purely based on the geometry of the particles. However, these models do not suggest methods for overcoming the problem of particles agglomerations.
As an example, EP 0 042 935 B2 describes, and thus it is known in the art, that if silica fume particles are aggregated, some sort of dispersing action, such as grinding, may be applied. Accordingly, for silica fume to be utilized as an effective ultrafine material mixed with inorganic binders such as Portland cement, there are two issues that should be addressed. Firstly, the agglomerations should be broken down, and secondly, the silica fume should be distributed uniformly throughout the concrete.
In attempts to address the problem of silica fume particle aggregation, many modern cement manufacturers have experimented with dry blending silica fume with cement using various mills and blenders. The experience of these manufacturers and also numerous experiments of authors on mixing dry cementitious composition on various types of intensive blenders demonstrated inability of these blenders to de-agglomerate silica fume to a sufficient degree, and thus provide maximum packing density of the multi-component cementitious mixture. Breaking down silica fume with cement in the process of cement milling has proved beneficial; however cement milling is generally performed in a factory utilizing a process of clinker milling which produces large batches for commercial use. One disadvantage of dry mixing of silica fume in the process of cement manufacturing is a lack of production flexibility. Specifically, it is impractical to produce relatively small and diverse batches of blended hydraulic cement according to requirements of dissimilar customers. Another drawback is that a limited amount of silica fume can be added into cement in the process of its manufacturing. Silica fume is normally added at a range between 3 and 10% by weight, and almost certainly under maximum 15% defined by many national standards, and particularly by National Standard of Canada CAN/CSA-A3000-13. This quantity of silica fume is generally below its optimum amount (up to 25% in many UHPC compositions and higher in some) required for making blended cement compositions with maximum packing density. Besides, the milling cannot be used for optimum packing of multi-component mixtures, containing in addition to silica fume, other supplementary cementitious material like fly ash, slag, etc., and non-cementitious fillers like quartz powder, sand, etc. Another reason the milling cannot be used for optimum packing of multi-component cementitious mixtures is that it is considered impossible to calculate optimum ratio of the components based on their original sizes because of comminution of different components to varying and unpredictable degrees in the process of milling.
Recently, nano-sized additives including, for example: nanosilica, nano-clay, nano-TiO2, nano-Fe2O3, carbon nano-tubes and fibers, are being utilized in the production of cementitious materials. However, it is known in the art that powder cohesion increases with decreasing size of the particles, and accordingly de-agglomeration of the nano-particles and their uniform distribution in the cementitious matrix is an even more challenging problem than in the case of the ultrafine micro-particles. The nano-sized additives may be supplied in the form of suspension like nanosilica, or as a powder such as nano-TiO2, carbon nano-tubes, and nano-fibers. In the case of powdered nano-tubes, the nano-tubes are dispersed before adding them into a cementitious mixture. Dispersion of the nano-tubes is carried out by methods that utilize high dispersing energy, including ultracentrifugation and sonication (U.S. Pat. No. 8,865,107; US 20090229494). The dispersed nano-tubes, according to U.S. Pat. No. 8,865,107 and US 20090229494 may be subsequently mixed with cementitious materials using a standard mixer. However, such standard mixers, having low dispersion energy, are unable to provide complete uniform distribution of nano-sized particles or fibers in the cementitious matrix.
The optimal usage of a superplasticizer along with optimization of the dry binder mix, are among the most important factors defining strength and workability of UHPC. Superplasticizers, which are also known as high range water reducers, are chemical admixtures used where well-dispersed particle suspension is required. The addition of superplasticizers to mortar or concrete allows for the reduction of the water to cement ratio, without affecting the workability of the mixture and thus improves the performance of the fresh paste and the hardened concrete object. One efficient usage of superplasticizer is provided by Super-High-Strength High Performance Concrete, Pu Xincheng, 2013 by Taylor & Francis Group, LLC, wherein the superplasticizer is adsorbed onto the surface of the cement particles, avoiding the adsorption of a large part of superplasticizer by the aggregates, thus reducing the plasticizing effect. Furthermore, EP0010777 discloses introduction of the superplasticizer as a dry powder to the dry mix before adding water as a more efficient way of utilizing superplasticizer.
A number of prior art references disclose methods of improving dispersion by introducing superplasticizer into cement dry mix. For example, EP0696262 discloses a method for producing cement comprising mechano-chemical treatment of a mineral-polymeric or a mineral mixture of Portland cement and a SiO2-containing microfiller, such as silica fume and/or powdery water reducing agents of melamine or naphthalene type in milling equipment. RU 2577340 discloses combining powdered superplasticizer with cement and WO 2009084984 and RU2371402 disclose merging dry superplasticizer with cement. Furthermore, it has been taught by Sobolev et al. in Nanomaterials and Nanotechnology for High-Performance Cement Composites, Proceedings of ACI Session on “Nanotechnology of Concrete: Recent Developments and Future Perspectives” Nov. 7, 2006, Denver, USA, pp. 91-118 that combining superplasticizer with cement by intergrinding cement and dry modifiers in a high-energy mill serves the purpose of attaching the superplasticizer organic functional groups to the surface of inorganic Portland cement and forms organo-mineral nano-layers on the surface of cement. Specifically, WO 2014148944 coined the term “nano-cement” to refer to the formation of a nano-layer having a thickness from 20 to 100 nm of melted dry superplasticizer around the cement grain.
All of above-mentioned references are directed towards the superplasticizer being either melamine or naphthalene; the state of the superplasticizer matter being solid or powdered; the equipment being mills, specifically ball mills; and the processing stage being directed towards cement milling. These limitations lead to a number of disadvantages. Firstly, the use of melamine or naphthalene superplasticizers has been universally replaced by polycarboxylate type water reducers. The requirement for these superplasticizers to be powdered further aggravates the problem, since most of the superplasticizers on the market are liquids. Another drawback of the direct addition of the melamine or naphthalene superplasticizers on the cement grains is the observed chemical effects perturbing the action of superplasticizers. Specifically, an organo-mineral phase can form around cement particles at the early stages of hydration consuming superplasticizers in an unproductive way. It is known in the art that part of the added superplasticizer can be intercalated in diverse hydration products and this fraction is no longer available for dispersing cement agglomerates (Flatt, R. J., Houst, Y. F., A simplified view on chemical effects perturbing the action of superplasticizers, Cement and Concrete Research 31 (2001) 1169-1176). However, yet another issue is the fact that methods of applying a superplasticizer in the process of cement manufacturing is limited by use of only one kind of chemical additive, namely polymer water reducing agents, and excludes use of other chemical additives such as hydration stabilizers, accelerators and others that come in liquid form.
U.S. Pat. Nos. 5,709,743 and 7,041,167 utilize finely ground calcium silicate hydrate (C-S-H) as a setting and hardening accelerant and strength enhancing additive for cementitious products. More recent research by Land G and Stephan D, Nanoparticles as accelerators for cement hydration, Ultra-High Performance Concrete and Nanotechnology in Construction, Proceedings of Hipermat, Kassel, Mar. 7-9, 2012, pp. 112-118 discloses influence of nanoparticles on the kinetics of cement hydration and their potential to substitute conventional setting and hardening accelerators, as well as their undesirable side effects. It is taught that nano-particles, and above all nano-C-S-H seeds, added into fresh cement paste, stimulate the nucleation process during early cement hydration, accelerate setting and hardening, and result in enhanced mechanical properties of the hardened cement paste. However, the manufacturing of sub-micron and especially nano-sized C-S-H seeds is costly and involves complex processing, which results in extremely high prices of such hardening accelerating admixture for concrete (such as that produced by BASF™ as Master X-Seed™ 100) in comparison with known accelerating additives. Another drawback of using C-S-H as an accelerant is that it must be added as an aqueous slurry. This is a disadvantage for UHPC mixes, since the addition of water contained in the C-S-H slurry can impact the required low water to binder ratio. Recent research by Moghaddam, Sakineh E. et al. propose the synthesis of C-S-H in small scale lab conditions, not on pilot or production scale, via an in situ generated seed-mediated growth that is much more expensive than the Master X-Seed™ 100 and is presently in early stages of research (Moghaddam, Sakineh E., Vahid Hejazi, Sung Hoon Hwang, Sreeprasad Sreenivasan, Joseph Miller, Benhang Shi, Shuo Zhao et al. “Morphogenesis of cement hydrate.” Journal of Materials Chemistry A (2017). Doi: 10.1039/C6TA09389B).
With all the diversity of known mortar and concrete compositions and methods of their making, the processes for preparing shaped concrete elements and structures from these compositions can be roughly described by a basic process with some slight variations, namely mixing dry cement compositions with sand, aggregates, water and chemical additives, casting shaped elements and structures and hardening the subjects. The workability of the paste and fresh concrete, and quality of the hardened cementitious subjects, is determined by the quality of mixing process, and more specifically by the extent to which the blending process can provide uniformity of distribution of the mixture components and deliver water and chemicals to each cement grain. Specifically, it is known in the art that the mixture performance is a quantity of homogeneity and thus a “quality criteria”. Indeed, Dehn, F. in Influence of Mixing Technology on Fresh Concrete Properties of HPFRCC, Proceedings of Int'l RILEM workshop on HPFRCC in structural applications, Honolulu, USA, published by RILEM SARL, 2006 teaches that mixing processes have an emphatic influence on the obtainable concrete properties in the fresh state and that the efficiency of a mixer is determined by the homogeneity of the concrete produced.
There has been extensive research on processing cementitious pastes in various types of mixers, including drum mixers, pan mixers, plaster/mortar mixers, planetary mixers and intensive Eirich mixers, to study the workability of pastes and various properties of hardened concrete such as density, strength, degree of homogenization of various components in the concrete, cement hydration dynamics, and others. It is known in the art that properties of concrete are improved by increasing the intensity of the mixing process. However, there are intrinsic limitations with such existing mixing technologies. Specifically, existing mixing technologies do not achieve the highest possible potential strength of the hardened cementitious subject. Increasing intensity in the existing mixing technology improves components homogenization and delivery of water and chemical additives to these components to a certain limited mixing intensity, after which, further increases in mixing intensity leads to de-mixing and component separation, which in turn results in decreased concrete strength.
Mixture of cementitious material is heterogeneous, containing different materials with a wide range of particle sizes, densities, shapes, etc. Considering a continuum of cementitious material comprising three major product groups: cement paste (cement-water system), mortar (cement-water-sand system), and concrete (cement-water-sand-aggregates system), it may be noticed that the rheological properties of these groups are significantly different. Accordingly, each of these group of materials requires its optimum energy for providing the best homogenization. Existing mixing technologies deliver generally the same energy to all different components, thus are unable, regardless of the mixer type, to supply the specific required energy to a particular group of the components, and accordingly cannot provide the best possible mixture homogenization. Another drawback of the existing mortar/concrete mixing technology is a common workflow with water addition to dry mix, which causes liquid enriched lumps or agglomerates at the moment of water contacting powder, and subsequent highly energetic mechanical treatment with plenty (sometimes extra amounts) of water addition is required for the lumps to be homogenized, and for water to be delivered to the majority of cement particles. Finally, the problems with existing mortar/concrete process mixing technologies are magnified by the fact that though the designers of mixers appear to recognize that there may be distinctive differences in rheology between cement, mortar and concrete mixtures, there is no explicit mathematical model which determines the rheology of each of these mixtures and defines the optimal mixing parameters on the basis of such a model.
In addition to problems related to wet mixtures, there are recognized problems pertaining to dry cementitious mixtures containing Portland cement and other fine and highly hygroscopic components such as silica fume. These problems include but are not limited to: reduced and limited shelf life, excess dusting which can be hazardous, and the segregation of the components in the process of transportation and handling of the mixtures. An important property of cement is shelf life which is dependent on the ability to maintain its chemical activity during prolonged storage. Cement is a hygroscopic material, and in the presence of moisture it undergoes hydration. Absorption of moisture from the air causes the irreversible chemical interaction of cement with water. As the result, under the influence of moisture and carbon dioxide present in the atmosphere, cements lose activity in storage.
Portland cement is designed to react chemically with water and any exposure to the surrounding moisture will cause it to set and harden. Depending on the storage conditions of the cement the possible loss of its activity amounts to about 15% per month. The universal recognition of the inevitable deterioration of cement during transportation and storage explains its limited guaranteed shelf life being generally not more than 6 months, and in many cases cement only remains useful for a period of 3 months or less from delivery. Dry cementitious mixtures containing, in addition to Portland cement, other highly hygroscopic materials such as silica fume, are even more problematic in terms of limited shelf life, such materials include known UHPC's dry mixtures. As such, existing recommendations for storage of Portland cement and its dry mixtures suggests the isolation from surrounding atmosphere by using airtight multilayer plastic bags, dehumidified storage rooms with minimized air circulation, and stacking cement away from walls, etc.
A number of prior art references have attempted to improve the shelf life of cements by utilizing alternative packaging techniques. For example, CN101428992A and CN102485687A describe a dry powder mortar packaged in grouping manner and a two-component packaged cement, where the mortar components with different shelf lives are packed separately so that the component with the shortest shelf life (generally Portland cement) would not negatively affect the shelf life of the whole mixture. This solution may preserve the activity of some cement components until mixed, e.g. sand from deterioration, however these solutions do not extend the shelf life of cement. Once the components are mixed, the shelf life of the cement mixture is limited by the component with the shortest shelf life. CN 103420648A teaches a method for postponing cement solidification and prolonging expiration date, where the cement and dried sand are evenly mixed with a weight ratio of 1 to 4 and then the mixture is packaged in two layers. The method requires that part of the ambient moisture be absorbed by dry sand, thereby protecting cement from premature hydration. However, the fact is that cement is more hygroscopic that sand, and as such the method is ineffective and cannot be universally applied to various mixtures of cement and sand. Besides, such cement to sand ratio is not used in UHPC mix design.
Other prior art references have attempted to improve the shelf life of cements by pelletization. For example, U.S. Pat. No. 2,221,175 describes pelletizing Portland cement to improve the cement for storage purposes and to eliminate a dust nuisance with use of water soluble binders or/and water insoluble binders. There are several problems with this pelletization method. In the case of the water insoluble binder, the granules/pellets should be re-grinded before mixing with water to the size of the original cement grains, since cement should be in the state of a fine powder in order to act as a hydraulic binder when mixed with water. Furthermore, the hardened water insoluble binder would reduce activity of the cement by covering part of the cement grains and creating obstacles for making a hardened monolithic smooth cement matrix. In the case of the water soluble binder, the water in the binder would cause hydration of the cement and create hydrated cement granules, which would reduce the cement activity and require re-grinding of the cement containing hydrated agglomerates. U.S. Pat. No. 8,992,679 describes dry construction pellets comprising uncured cement powder and a non-reacting binder which may be a small amount of water not sufficient to totally cure the cement. However, the pellets according to U.S. Pat. No. 8,992,679 have essentially the same problems as the above mentioned U.S. Pat. No. 2,221,175. Furthermore, the small amount of water described in U.S. Pat. No. 8,992,679 is not sufficient to fully cure the cement. Rather, the amount of water disclosed would cure the cement partially, thereby reducing partial activity of the cement. Another shortfall of the dry pellets according to the U.S. Pat. No. 8,992,679 is that while comprising cement, sand and gravel, the dry pellets lack any essential UHPC supplemental cementitious materials (SCM) like silica fume, fly ash, etc. Additionally, neither U.S. Pat. No. 2,221,175 nor U.S. Pat. No. 8,992,679 disclose that the cementitious materials contain reinforcing micro-fibers.
It is known it the art that the efficiency of reinforcing fibers is defined by the strength of the fibers themselves and by the bonding strength between the fibers and the concrete matrix. Generally, smooth metal fibers have poor bonding to cementitious matrix. There are two known methods in the art of improving this bonding slip performance, the first by utilizing fibers having some special shape (U.S. Pat. Nos. 4,960,649 and 5,981,630), and the second method involves either treating the surface of the fibers with chemical treatments such as strong acids which etch the surface or by the deposition of mineral compounds on the fibers (U.S. Pat. Nos. 6,955,844; 6,478,867).
Finally, U.S. Pat. No. 4,031,184 relates to a process of reclaiming cement kiln dust and U.S. Pat. No. 4,341,562 teaches kiln dust pellets or cement kiln dust and fly ash pellets for making a lightweight concrete. These patents propose pelletizing cement for making non-dissolvable aggregates.
It is an object of the following to obviate or mitigate at least one of the foregoing disadvantages.